The present invention relates to hydrogen phosphide-releasing compositions, especially to pest control agents, comprising aluminium phosphide.
For purposes of pest control and protection of stored agricultural commodities preparations based on hydrolysable phosphides of alkaline earth and/or earth metals such as calcium, magnesium or aluminium phosphide are employed to an increasing extent. These are caused by atmospheric humidity or the moisture content of stored commodities to decompose into highly toxic phosphine and inert oxides/hydroxides. The liberated phosphine may auto-ignite under certain unfavorable conditions, for which reason special expedients are necessary to permit the application safely and without hazard. In addition it is generally desired to so regulate the hydrolysis rate of the phosphides that the fumigation workers are not exposed unnecessarily to toxic gas concentrations.
Substances which thermally decompose to liberate NH.sub.3 and/or CO.sub.2 such as ammonium bicarbonate, ammonium carbonate, ammonium carbamate etc. have been used successfully for suppressing the tendency to auto-ignite of phosphine formed hydrolytically from phosphides. More recently other inhibitors have been described which even in low concentrations inhibit or retard the spontaneous reaction between phospine and atmospheric oxygen. It is also known to admix to the phosphide readily volatile organic substances, the heat of evaporation of which causes the phosphide particles to be surrounded by a cooled zone, thereby preventing heat accumulation which might result in ignition of the phosphine.
These measures have very little or no effect on the violent rate of formation of the phosphine which results when the metal phosphide comes into contact with liquid water. To suppress this reaction with liquid water it has been proposed to hydrophobise the phosphides or phosphide particles. Paraffins, waxes, stearates, silicones, synthetic resins etc. have been described as hydrophobing agents. See, e.g., U.S. Pat. No. 3,132,067; 3,372,088; Fed. Rep. of Germany DOS 27 05 228; and U.S. Application Ser. No. 964,410, filed Nov. 28, 1978, now abandoned, whose disclosure is incorporated herein by reference, and the references cited therein. It is also known that the individual phosphide particles or groups of particles are coated with a water-repellent dense coating which is claimed to prevent hydrolysis such that a bursting substance is added which is said to burst open the coating and thus trigger the reaction with moisture after a time delay.
It was furthermore known to depress the reactivity of metal phosphides against liquid water in pest control; agents by blocking the hydrophilic centers of the phosphides with water-insoluble metal soaps, in particular stearates. The expressly described purpose of this procedure was the abolition of previously known auto-ignition inhibitors, in particular ammonia salts such as ammonium carbamate. This procedure was equally effective for calcium phosphide, magnesium phosphide as well as aluminium phosphide. For various commercial and practical reasons aluminium stearate (and to a lesser extent magnesium stearate) were selected as the preferred blocking agent and even used on a commercial scale in certain countries by the present applicant.
Practical experience and experiments have shown that the above described teaching may at the most serve to maintain a reasonable control over the hydrolysis of alkaline earth and earth metal phosphides caused by atmospheric humidity. If phosphide containing pest control agents prepared according to the state of the art, are contacted with liquid water, as may very well happen accidentally in practice, it is found that, depending on the ratio of water/preparation more or less severe steam generation and temperature rises to values even exceeding 100.degree. C. may result, demonstrating that the reaction cannot be controlled adequately. Under such conditions the admixed protecting agents are volatilised very rapidly so that they do not yield an adequate protective effect. Moreover, the measured reaction temperatures are at least in part within the range of the ignition temperature of phosphine which according to the literature is approximately 100.degree. C. In the experiments, the oxidation of the phosphine, i.e., the formation of P.sub.2 O.sub.5, smoke and sometimes even spontaneous ignition may be observed. It would be highly desirable--and this is an object of this invention--to mitigate this safety risk which should not be under-estimated in the practical context, and nevertheless to provide an easily manufactured and safely applied pest control agent based on hydrolyzable aluminium phosphide.